The retention sites and residence times for inhaled particles deposited in the non-ciliated pulmonary regions are important factors in the pathogenesis of major occupational respiratory diseases. This study is directed toward the quantitative characterization of the pathways, transport rates, and storage sites for respirable particles with defined size and shape and concentration. Particle retention within the thorax and tracheal lymph nodes following aerosol inhalation will be measured for at least 8 months in 4 donkeys by the external in vivo dectection of radionuclides within the particles, and for ferromagnetic particles, by remanent magnetism.